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Fengnan Chang <changfengnan@bytedance.com> says: iomap: add simple dio path for small direct I/O When running 4K random read workloads on high-performance Gen5 NVMe SSDs, the software overhead in the iomap direct I/O path (__iomap_dio_rw) becomes a significant bottleneck. Using io_uring with poll mode for a 4K randread test on a raw block device: taskset -c 30 ./t/io_uring -p1 -d512 -b4096 -s32 -c32 -F1 -B1 -R1 -X1 -n1 -P1 /dev/nvme10n1 Result: ~3.2M IOPS Running the exact same workload on ext4 and XFS: taskset -c 30 ./t/io_uring -p1 -d512 -b4096 -s32 -c32 -F1 -B1 -R1 -X1 -n1 -P1 /mnt/testfile Result: ~1.92M IOPS Profiling the ext4 workload reveals that a significant portion of CPU time is spent on memory allocation and the iomap state machine iteration: 5.33% [kernel] [k] __iomap_dio_rw 3.26% [kernel] [k] iomap_iter 2.37% [kernel] [k] iomap_dio_bio_iter 2.35% [kernel] [k] kfree 1.33% [kernel] [k] iomap_dio_complete Introduce a simple dio path to reduce the overhead of iomap. It is triggered when the request satisfies all of: - a READ request whose I/O size is <= inode blocksize (fits in a single block, no splits); - no custom iomap_dio_ops (dops) registered by the filesystem; - no caller-accumulated residual (done_before == 0); - none of IOMAP_DIO_FORCE_WAIT / IOMAP_DIO_PARTIAL / IOMAP_DIO_BOUNCE set, the range is within i_size, and the inode is not encrypted. The bio is allocated from a dedicated bioset whose front_pad embeds struct iomap_dio_simple, so the whole request lives in a single cacheline-aligned allocation and no separate struct iomap_dio is needed. Completion is handled inline from ->bi_end_io for the common success case, and only punted to the s_dio_done_wq workqueue on error. After this optimization, the heavy generic functions disappear from the profile, replaced by a single streamlined execution path: 4.83% [kernel] [k] iomap_dio_simple With this patch, 4K random read IOPS on ext4 increases from 1.92M to 2.19M in the original single-core io_uring poll-mode workload. Below are the test results using fio: fs workload qd simple=0 simple=1 gain ext4 libaio 1 18,740 18,761 +0.11% ext4 libaio 64 462,850 480,587 +3.83% ext4 libaio 128 459,498 478,824 +4.21% ext4 libaio 256 459,938 480,156 +4.40% ext4 io_uring 1 18,836 18,880 +0.24% ext4 io_uring 64 568,193 600,625 +5.71% ext4 io_uring 128 570,998 602,148 +5.46% ext4 io_uring 256 572,052 602,536 +5.33% ext4 io_uring_poll 1 19,283 19,272 -0.06% ext4 io_uring_poll 64 989,735 1,013,342 +2.39% ext4 io_uring_poll 128 1,467,336 1,538,444 +4.85% ext4 io_uring_poll 256 1,663,498 1,830,842 +10.06% xfs libaio 1 18,764 18,776 +0.06% xfs libaio 64 462,408 480,860 +3.99% xfs libaio 128 461,280 480,819 +4.24% xfs libaio 256 461,626 480,190 +4.02% xfs io_uring 1 18,871 18,903 +0.17% xfs io_uring 64 570,383 597,399 +4.74% xfs io_uring 128 568,290 597,370 +5.12% xfs io_uring 256 570,616 598,775 +4.93% xfs io_uring_poll 1 19,211 19,315 +0.54% xfs io_uring_poll 64 989,726 1,008,455 +1.89% xfs io_uring_poll 128 1,430,426 1,513,064 +5.78% xfs io_uring_poll 256 1,587,339 1,742,220 +9.76% * patches from https://patch.msgid.link/20260701033253.46420-1-changfengnan@bytedance.com: iomap: add simple dio path for small direct I/O iomap: pass error code to should_report_dio_fserror directly iomap: factor out iomap_dio_alignment helper Link: https://patch.msgid.link/20260701033253.46420-1-changfengnan@bytedance.com Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>
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